Heresy perhaps, but I have to ask, why bother carrying a knife while running?
If it's a bad dog you're worried about, the gas is a much better idea. Or a stick. Next time you see a serious runner carrying a length of bamboo or another light stick, pull up alongside and ask to hear his dog attack story. I think it's naive to think you can outrun a bad dog or fend him off with a knife without getting mangled.
As for bad guys, my personal experience has been that's it's just not a problem. Perhaps others have different tales, but I've been running marathons for 25 years. I cover a lot fo miles, about a quarter of them in unfamiliar places due to work travel. I've never had an encounter with a thug where I wished I had a knife. I mean, if you're a lowlife street criminal, it's going to seem like an awfully lot of work to catch up to a fast-moving guy in his underwear, and what's he likely to have that you'd even want?
I'm surprised at some of the suggestions for carrying folders. I've found even my lightweight Spyderdo Delica flops around in the waistband in an annoying way. Only thing I've ever used that didn't interfere with my revery was a little Outdoor Edge Wedge slipped handle-down outside my pants and covered by a shirt. But the glove suggestion is on the money--you can carry something small inside the gloved hand and not be troubled by motion. And the adjustable clip suggestion sounds well worth checking out.